When operating the refining of steel by blowing pure oxygen from the top downwards by means of a lance, it is advantageous to be able to assure swirling of the metal bath and of the slag, which is effected by blowing a swirling gas at a low flow rate from the bottom upwards through the base of the converter.
Different systems are known for blowing a swirling gas at a low flow rate through the bottom of a converter.
Some use porous refractories, the open and, possibly, orientated porosity of which enables gas to be channelled under pressure through the refractory mass.
Others use permeable elements, consisting of compact refractory bricks, the joints between bricks forming very narrow passages which, nevertheless, are permeable to gases under pressure. These permeable passages are produced either by a designed assembly of metal plates surrounding each of the bricks or by moving together bricks, which have been previously sawn and to both sides of which very thin metal wires have been attached.
However, all these known systems show significant and variable rates of wear on the refractories. For the rates of wear to be acceptable in the industrial application of these known systems, on the one hand, the temperature of the metal bath has to remain moderate, for example always below 1660.degree. C., and, on the other hand, the working methods have to be such that the permeable elements are permanently covered with a deposit of magnesia-enriched slag, by a so-called "buttering" technique. It follows from this that, below 1660.degree. C., reliability of the permeable elements is not necessarily secured if the deposit of magnesia slag is insufficient and that, above 1660.degree. C., the permeable elements can hardly be used, because this deposit of magnesia slag is formed with even greater difficulty.